Added support for Rain Gauge

Added mapping for Rain measurement in SENSOR_PROPERTIES and updated
usage documentation.

Running samples/printAllLastData outputs:
```
Rain Gauge
                     rf_status : 60
                    battery_vp : 6402
                          When : 06:39:45
                          Rain : 0
Indoor
                         Noise : 34
                   Temperature : 21.2
                          When : 06:39:53
                      Humidity : 31
                      Pressure : 1035
                           Co2 : 542
Indoor 2
                           Co2 : 468
                     rf_status : 63
                   Temperature : 23
                          When : 06:39:38
                      Humidity : 27
                    battery_vp : 5882
Outdoor
                     rf_status : 59
                    battery_vp : 6132
                          When : 06:39:51
                   Temperature : 0.3
                      Humidity : 61
```

lnetatmo.py

@@ -53,6 +53,7 @@
         'Humidity' : 'b',
         'Noise' : 'S',
         'Pressure' : 'e',
+        'Rain' : 'f',
         'When' : 'K'
         }
 

usage.md

@@ -250,7 +250,7 @@ Methods :
     * Output : Sensors data dictionary (Key is sensor name)
 
      AT the time of this document, Available measures types are :
-      * a full or subset of Temperature, Pressure, Noise, Co2, Humidity, When (measurement timestamp) for modules including station module
+      * a full or subset of Temperature, Pressure, Noise, Co2, Humidity, Rain, When (measurement timestamp) for modules including station module
       * battery_vp : likely to be total battery voltage for external sensors (running on batteries) in mV (undocumented)
       * rf_status : likely to be the % of radio signal between the station and a module (undocumented)